1. Field of the Invention
The present invention relates to nuclear magnetic resonance (NMR) material analysis, and more specifically to a method and apparatus for determining the amount of a material having a first molecular species, such as an organic material, in a mixture containing another molecular species, such as an aqueous material, wherein both molecular species have a common atomic species, such as hydrogen.
2. Description of the Prior Art
There are many situations in a variety of industries, ranging from research activities to production material measurements, and the like, wherein it is necessary, or at least very desirable, to be able to determine the amounts of various materials or molecular species in a mixture. For example, it is often desirable to be able to determine and measure quickly the relative amounts of organic and nonorganic materials in a mixture such as for quality control or for recording the amounts of such materials present in the mixture as a basis for production records, sales of materials, production performance and cost/benefit analyses, as a basis for royalty payments, and the like.
One such industry in which there has been a need for decades for a practical, reliable, yet high quality and accurate method and apparatus for distinguishing and measuring the amount of organic material in a mixture is the oil industry. Crude oil is typically produced from wells drilled into geological formations that also contain various amounts of salt water and other impurities. In practice, it is difficult, and in most situations impossible, to produce crude oil from the wells without also producing substantial quantities of salt water along with the oil. Typically, such production includes a mixture of crude oil, gas, and salt water, along with varying smaller amounts of other impurities, such as sand, clay, sulfur, salt, minerals, and other materials, depending to some extent on the character and content of the formation. Such mixtures flow typically in a two-phase, or sometimes even three-phase, flow that is seldom uniform or homogeneous and can vary in proportions over time, sometimes in an erratic manner. Yet, there has been for many years a great deal of need to be able to determine and measure the exact amount of crude oil being produced by a well, both on a continuous basis and on a spot or instantaneous basis.
The simplest method of determining amounts of oil in a mixture of materials produced by a well is to separate the oil from the gas, water, sand, mud, and other materials in the mixture and then measure it. The fraction of the oil in the total mixture, i.e., the oil cut of the mixture, can be determined by comparing the volume of oil to the volume of the total mixture. However, conventional processes for separating oil from the other constituents of the well production mixture require substantial time, equipment, and effort and does not provide a measurement of oil cut on a real time continuum basis.
There are several types of oil-cut meters available, the better known of which measure oil-cut of the well production by determining the reflection or transmission of radio frequency (rf) electromagnetic waves. However, such meters are really water-cut meters, instead of oil-cut meters. Water is electrically conductive, while oil is a dielectric medium. Such meters determine water-cut as a function of varying conductivity and dielectric constant of the mixture, then subtract the water cut from the total to determine oil cut. One of the basic fallacies or margins of error for this approach is that it assumes that whatever is not water in the mixture is oil. That assumption classifies as oil all other impurities, such as sand, mud, and the like, in the mixture that is not water. Obviously it is not an assumption on which accurate data can be based.
Another basis for inaccuracy in such prior art meters is that the degree of reflection or transmission of the radio frequency waves varies as a function of the sizes and shapes of the water drops suspended in the mixture. Also, the conductivity of water varies sensitively as a function of dissolved ions, which can fluctuate significantly. Therefore, there is no direct one-to-one relationship between the actual wave reflection or conductivity measurement and the actual water-cut. Consequently, there is a substantial margin of error in the water-cut measurement itself even before it is subtracted from the total where the other potential for significant error is introduced by assuming incorrectly that all that is not water is oil, as described above. As a result, such prior art water-cut meters disguised as oil-cut meters are not very accurate or reliable and have only limited utility in the industry.
There is another meter available that also measures water-cut and gas in the well production mixture, wherein the cut ratio is obtained from the bulk density of the total mixture and the bulk density of a degassed sample. In this kind of so-called "three-phase flowmetering", a large error can be introduced either by the degassed sample not representing accurately the bulk flow, by the heavy impurities, such as mud and sand, or by fluctuations in the density of the oil produced in some oil wells. A small change in oil density can result in a large error because the density of oil is close to that of water.
Nuclear magnetic resonance (NMR) analysis can be a powerful tool in detecting and determining the amounts of aqueous materials (water or water based materials) in a mixture. It is known that hydrogen atoms in both the aqueous material and the organic material emit rf waves of the same Larmor frequency intrinsic to the hydrogen atom. However, the rf emission from the hydrogen atoms in the organic material decays much faster than the rf emission from the hydrogen atoms in the aqueous material.
For example, the spin-spin relaxation time of typical organic liquid, such as oil, is about 50 milliseconds while the spin-spin relaxation time of water is about three (3) seconds. Therefore, the initial maximum amplitude of the rf emission from an aqueous-organic mixture is the sum of the rf emissions from both the aqueous and the organic components. However, after about one-half (1/2) minute, the rf emission from the organic component has decayed or attenuated by about 99.9% and effectively disappeared so that a measurement of total rf emission at that time comprises essentially rf emission from the aqueous component only.
Then, the initial amplitude of the rf emission from the aqueous component only can be determined mathematically by utilizing the curve of decaying or attenuating rf emissions measured by a method called "spin echo" and extrapolating that aqueous component curve backwards in time to the initial rf emission start point. Once the initial amplitude of the rf emission of the aqueous component is obtained in this spin echo measurement/reverse time mathematical extrapolation manner, it can be subtracted mathematically from the actual initial measurement of the total rf emission of the mixture to determine the initial amplitude of the rf emission from the organic component. Finally, the amount of each component can be determined by comparing the initial maximum amplitude of the rf emission from each component with a measured standard rf emission of that component of known quantity.
While such conventional NMR analysis technique, as described above, can be used in the analysis of aqueous-organic mixtures, it suffers from several shortcomings. Specifically, this conventional NMR technique is based on the large difference between the respective spin-spin relaxation times of the aqueous and the organic components in the mixture, which phenomenon is very sensitive to temperature. Therefore, if the temperature of the mixture is not closely and accurately monitored and controlled, a large error can result in the process of extrapolating the initial maximum amplitude of the rf emission from the aqueous component. Also, the amount of aqueous component, i.e. water-cut, is determined first by the extrapolation from the rf emission, while the amount of organic component is computed by subtracting the extrapolated initial aqueous rf emission from the total initial rf emission. Therefore, like the water-cut meters disguised as oil-cut meters described above, this conventional NMR analysis method is really a watercut meter as far as the analysis of the aqueous-organic mixture is concerned. It does not detect and analyze the rf emission from the organic component in a direct method.
Therefore, there is still a substantial need in the industry, which has persisted for decades but never been met, for an oil-cut meter that can measure oil volume fraction directly from the oil in the mixture without relying on a water-cut valve. That need includes the requirement for an oil-cut meter that can operate and provide measurements on a continuous real-time continuum as well as on an instantaneous basis.